string引用计数的写时拷贝

string的各种实现：

#include <string>class String{public:String(char* str = "") :_str(new char[strlen(str) + 5]) { strcpy(_str+4, str); _str += 4; GetRefCount() = 1; } String(const String& s) :_str(s._str) {     GetRefCount()++;  }void Swap(String& s){    String* p;p = s._str;s._str = str;str = p;} ~String(){   if(--GetRefCount() == 0)   {       delete[](_str - 4);   }}int&  GetRefCount(){   return  *((int*)(_str - 4));}String& operator=(String s){    if(_str != s._str){   if(--GetRefCount() == 0)   {         delete[](_str - 4);   }       _str = s._str;      GetRefCount()++;}return *this;}const char* GetStr(){   return _str;}size_t Size(){   return  _size;}size_t Capacity(){   return _capacity;}void PushBack(char ch){if(_size >= _capacity){  Expand(_capacity * 2);}   _str[_size++] = ch;   _str[_size] = '\0';}void PushBack(const char* str){size_t len = strlen(str) + _size;    if(len >= _capacity){  Expand(_capacity * 2);}   while(*str)   {      _str[_size++] = *str;  str++;   }   _str[_size] = '\0';}void PopBack(){   _str[_size - 1] = '\0';   _size--;}void Insert(size_t pos, char ch){if(_size > _capacity){   Expand(_capacity * 2);}int end = _size;while(end >= (int)pos){   _str[end + 1] = _str[end];   end--;}_str[pos] = ch;_size++;}void Insert(size_t pos, const char* str){    if((_size + strlen(str)) > _capacity){   Expand(_capacity * 2);}int len = strlen(str);size_t i = _size;while(i >= pos){  _str[i + len]  = _str[i];  i--; }while(*str){   _str[pos] = *str;   pos++;   str++;}_size = _size +strlen(str);}void Erase(size_t pos, size_t count){size_t len = pos + count;    if(len >= _size){    _str[pos] = '\0';}else{   while(_str[len])   {   _str[pos] = _str[len];   len++;   pos++;   }   _size -= count;   _str[_size] = '\0';}}size_t Find(char ch) const{    if(_str == NULL){    exit(1);}char* cur = _str;while(*cur){    if(*cur == ch){   return (cur - _str);}cur++;}cout<< "字符串中没有此字符"<<endl;system("pause");exit(1);}size_t Find(const char* str) const{     char* cur = _str; char* src = str; if(_str == NULL) {    exit(1); } while(*cur) { char* p = cur;while(*src) {     if(*p == *src) {     p++;src++; } else {   break; }}         if(*src == '\0')    {    return sub -_str + 1;    }    cur++;    break; } cout<<"原字符串中没有此字符串"endl; system("pause"); exit(1);}char& operator[](size_t pos){   assret(pos < _size);   return _str[pos];}bool operator<(const String& s) const{    for(size_t i = 0; i < _size && i < s._size; ++i){  if(_str[i] < s._str[i])  {     return true;  }  else if(_str[i] > s._str[i])  {    return false;  }}      if(i == _size && i < s._size)  {     return true;  }  else  {     return false;  }}bool operator<=(const String& s) const{    return !(_str >s._str);}bool operator>(const String& s) const{      for(size_t i = 0; i < _size && i < s._size; ++i){  if(_str[i] > s._str[i])  {     return true;  }  else if(_str[i] < s._str[i])  {    return false;  }}      if(i < _size && i == s._size)  {     return true;  }  else  {     return false;  } }bool operator>=(const String& s) const{   return !(_str <s._str;}bool operator==(const String& s) const{ for(size_t i = 0; i < _size && i < s._size; ++i){  if(_str[i] > s._str[i])  {     return false;  }  else if(_str[i] < s._str[i])  {    return false;  }}      if(i == _size && i == s._size && _str[_size] == s.str[s._size])  {     return true;  }  else  {     return false;  }}bool operator!=(const String& s)const{  return !(_str == s._str);}void Expand(size_t n){   if(n > _capacity)   {     _str = (char*)realloc(_str, n+1); assert(_str); _capacity = n;      }}